[Me and Rich ...]
Failure to find strong evidence of ETI in any given SETI
program is not a failure of the program, it is a scientific
*result*.
R> I'm amazed at how many think that you can derive population
R> statistics from one example, the earth.
(Actually, in re-reading this, I just realized that one can derive
population statistics from this: The number of civilizations in the
Galaxy is at least 1.
R> We seem to have a different vision as to what population statistics
R> are.
While I agree that, at times, one can question whether Earth really
counts as having "civilization" or not, if you say that the Earth has
a civilization on it, then my statement is correct. Yes?
My view (maybe because I'm an astronomer rather than a demographer) is
that the most fundamental issue in population statistics is, Is there
are population?
The question that SETI is trying to answer is whether the number of
civilizations in the Galaxy is more than 1.)
R> Agreed.
O.k. So far, so good.
Re-read what Christopher wrote. He does not advocate deriving
the number of ET civilizations based on the Earth, but searching
for other ET civilizations to derive population statistics.
R> I'm referring to the Drake Equation, which many seem to think
R> proves the existence of anywhere from thousands to hundreds of
R> millions of ET civilizations in our galaxy alone. Is this not what
R> SETI is looking for?
Have I ever written that I think the Drake equation "proves" the
existence of ET civilizations?
R> Not where I have seen it (...). But it remains that it is usually
R> portrayed in this light.
Agreed, but I think you have to be careful in challenging SETI on
these grounds. I don't think that anybody in this discussion has
raised the Drake equation (or if it has been mentioned, it has been
fairly minor).
The Drake equation is really an illustration of our ignorance.
R> It illustrates nothing. It sheds no light.
Well, it illustrates things we don't know. We know very little about
the number of habitable planets, we know little about life's origins,
we know little about the evolution of life as it relates to the
development of intelligence, etc. To the extent that the Drake
equation might help us formulate questions which can then be the basis
of research, it might be useful. (Of course, those questions might
come up without the Drake equation at all, too. For instance, one
might be interested in planets from the standpoint of understanding
the late stages of stellar evolution.)
By the same token, we can already set simple limits on the number
of ET civilizations.
[...]
There are clearly no super-advanced civilizations
[...]
in our neighborhood radiating at huge power levels or we'd have
seen them already.
R> Exactly. And were they transmitting at much lower power in our
R> direction, we'd have seen that already.
Well, yes, but "lower" here is relative.
R> Agreed. But I note that Lou Sheaffer is posting right now that 1
R> watt would be sufficient. I suggest that this point is open to
R> discussion.
That's been answered elsewhere, but he's been posting numbers closer
to 1 MW.
If you'd like to read about an early attempt to set limits on the
number of civilizations in the Galaxy, visit the ADS and take a
look at the paper by Horowitz & Sagan (1993).
R> http://adsabs.harvard.edu/cgi-bin/bib_query?1993ApJ...415..218H
[...]
R> Interesting that they note "rare processor errors". Seems to be an
R> issue with seti@home as well.
Did you note the statement, "set limits on the prevalence of
supercivilizations transmitting Doppler-precompensated beacons at H
I or its second harmonic," which is really more relevant to the
question at hand?
R> Is it? I thought the question at hand was far more general. Above
R> you state it simply...
R> # The question that SETI is trying to answer is whether the
R> # number of civilizations in the Galaxy is more than 1.
Yes, and the Horowitz & Sagan paper suggests that there are no such
supercivilizations anywhere in the Galaxy (or even in nearby galaxies,
IIRC, at least near the H I line). That doesn't say anything about
mediocre advanced civs. that are pumping out only 1 GW or so.
Let me turn your statement around. How do you know that the
Galaxy contains no other civilizations, given that we know of one
civilization that can be detected over interstellar distances?
R> I make no claims to have such knowledge. I do claim that we just
R> don't know.
So why all of the posts to state something on which all of us
agree?
R> I don't see that this is the common message at all.
R> http://groups.google.com/groups?q=number+of+ET+civilizations&start=10&hl=en&lr=&ie=UTF-8&oe=UTF-8&scoring=d&selm=825fedc5.0404261920.358ffee9%40posting.google.com&rnum=12
Maybe I'm missing the obvious in this, but I don't know how this could
be seen as unreasonable. We know of many Jupiter-like planets, we
even know of a couple of Earth-mass planets. I don't think it's
outrageous to consider that there might be lots of Earth-mass planets.
If there could be Earth-mass planets, there might be Earth-like
planets harboring life, some with civs. on them. Most of the article
is just speculation on how advanced a civ. could become. Indeed, Kaku
says as much, "All this, in turn, will stimulate an active effort to
determine if any of them harbor life, perhaps some with civilizations
more advanced than ours." Nowhere in the article is there any
assertion that advanced civs. must exist, only that if they do that
they would have to obey the same laws of physics.
[And just a few other comments]
R> I'm not so sure about that. The dataset is still very small. It's
R> not clear, for example, that metal poor stars can have planets, or
R> perhaps planets not gas giants. There are whole populations of
R> stars that are metal poor, especially the populations of globular
R> clusters.
See the recent results on the planet detected around PSR
B1620-26, in the globular cluster M4.
R> http://www.extrasolar.net/planet.asp?PlanetID=30 Mass: 2.5 Jupiters
R> Seems like a gas giant near as I can tell.
[...] Well, yes, but gas giants are planets, too. You stated,
"It's not clear [...] that metal poor stars can have planets, ...."
We have one counterexample.
R> It's disturbing that you cut the sentence in half to delte the part
R> where I said...
R> "or perhaps planets not gas giants."
[...]
R> You don't have a counter-example as I specifically and clearly
R> excluded gas giants.
My apologies that you thought I was trying to distort your post, but I
did leave the whole sentence in my post. I just didn't see the
"perhaps planets not gas giants" as being very clear nor very
specifically excluding gas giants.
R> As for earth-type planets, we can't detect them yet. But would you
R> expect that they would be found around stars in globular clusters?
I don't know. I do know that if we don't look for them, we'll
never be able to say.
R> It's not clear that even were we to resolve such a planet, we would
R> be able to say anything about ETI's. But what reason is there to
R> expect terrestrial planets around metal poor stars?
Oh, I suspect we'd almost certainly not be capable of resolving it, at
least nowhere in the near future. Of course, we need not be able to
resolve it to get a spectrum of the planet, which might tell us
something about its atmosphere and the possibility of life.
Regarding whether I expect terrestrial planets around metal poor
stars, I'm inclined to think that we will not find terrestrial planets
in globular clusters. (Of course, then again, there were not supposed
to be pulsars in globular clusters, nor planets around pulsars, nor
Jupiter-like planets close to their host stars.)
As for the more general question of planets around metal poor stars in
the Galactic disk, I'm more agnostic. I'm worried that the current
surveys are still affected by selection effects, potentially some of
them unknown. Frankly, I'd like us to be in the position of trying to
figure out what the lowest metallicity star is with terrestrial planets.